Relationship Between Tropical Cyclone Activities Over Coastal Waters Of China And Interannual Variation Of Eastern Asian Subtropical Monsoons

In the context of scientific findings regarding a large-scale E–W negative correlation of the interannual variation of summer instantaneous temperature fields in the middle– upper troposphere over the Asia and the northern hemisphere Pacific region, referred to as Asian– Pacific Oscillation (APO) hereinafter, the paper is devoted to summer APO characterized quantitatively intensity of summer monsoons at Eastern Asian subtropics (EAS). A systematic analysis is also performed about the autumn and winter APO, as well as their relationships with the atmospheric circulation and temperature/pressure fields of the winter monsoons at Eastern Asian subtropics, respectively. Results show that the strength of autumn/winter APO can quantitatively represent the interannual variation of winter Eastern Asian Subtropical Monsoon intensity.From the statistical differences between annual frequencies of tropical cyclones (TC) given by the best track datasets of Joint Typhoon Warning Center (JTWC), Japan Meteorological Agency (JMA) and Shanghai Typhoon Institute (STI), the 1976-2005 JTWC TC optimal tracks and related NCEP/NCAR reanalysis monthly mean data are utilized for study by means of both correlation and composite analysis, by which to investigate, separately, the relationships between the APO intensity and Eastern Asian-western North Pacific air-sea environment in summer and autumn/winter, with the linkage explored of APO vigor with TC activities over Chinese coastal waters and TC distribution over western North Pacific. In view of the close association between autumn/winter APO strength and El Ni?o ( or La Ni?a ) events, CAM3.0-modeling is performed mainly forced by sea surface temperature over tropical Pacific, with dominant conclusions given below.1) There is an intimate relation of the TC activities over the coastal waters to APO intensity either in summer or autumn/winter on a synchronous basis. In summer when APO is strong (weak), the western North Pacific TC activities are largely west- (east-) and north- (south-)ward of mean, so that the offshore TC increases (decreases) greatly in frequency over eastern Chinese coastal waters. In September– October and November– December when intense (feeble) APO is occurring, western North Pacific TC activities are mostly west- (east-) ward of average, resulting in the Chinese offshore TC increasing (decreasing) in number.2) Summertime APO impacts the offshore TC activities and distributions by altering the vertical shear of zonal winds, low-level convergence and convection over coastal waters of China. For intense (weak) APO years, the EAS summer monsoon is strengthened (reduced), and there emerge anomalous cyclonic (anticyclonic) circulations and reinforced (diminished) convections in the lower troposphere over the coastal waters of East China, as well as declining (intensifying) vertical shear of troposphere zonal winds, thereby providing favorable (unfavorable) condition for the maintenance and development of the offshore TC activities.3) Summertime APO intensity bears a close relation to the vigor and position of monsoon trough over the South-China Sea (SCS)– tropical western Pacific (TWP) and to the direction of steering airflows, i.e., they determine the TC annual frequency in Chines offshore waters by affecting the location of the TC concentration area and their tracks. When APO is strong (weak), the SCS-TWP monsoon trough is positioned north-(south-) and west-(east-)ward of mean, and the concentration belt of TWP TC activities is west- (east-) and north- (south-)ward of mean, thus responsible for an increased (decreased) frequency of TC in eastern coastal waters of China. Also, in the years of higher (lower) APO strength, the TWP subtropical high is north-(south-) and east-(west-)ward of mean, with the easterlies on its south side diminished (enhanced), consequently favorable for TC moving in the northwest or west by north (south) to cause more (fewer) TC to come into the coastal waters of East China.4) In autumn and winter, there is a close association of the APO intensity with the atmospheric circulation and temperature/pressure fields of the EAS winter monsoons. In September– October when APO is strong, the EAS winter monsoons are weakened, indicated by the circulation as zonal straight flows over middle-high latitudes of East Asia, declined East Asia deep trough, subtropical high north- and eastward of mean, as well as decreased Hadley cell (Walker circulation) over East Asia continent and Low-latitude waters (subtropical areas of the northern hemisphere). On the other hand, in November– December when APO is weak, the EAS winter monsoons are strengthened, demonstrated by the circulation as intensified East Asia deep trough expanding to the south, decreased subtropical high eastward of mean, and significantly strengthened Hadley cell (Walker circulation) over East Asia continent and equatorial waters (tropical Pacific).5)In early autumn (September– October) the APO intensity affects offshore TC activities through a possible mechanism to alter the zonal wind vertical shear, low-level convergence and convective fields as the local environment over coastal waters of China , as well as west Pacific subtropical high's position, intensity and steering flow's direction as the large-scale circulation pattern. For strong (weak) APO the subtropical winter monsoon is decreased(increased) while the subtropical high is enhanced (weakened), with the high's ridge line extending (retreating) more westward (eastward), and the troposphere of middle-lower latitude western Pacific is under the control of anomalous easterly (westerly) wind, all these are favorable (unfavorable) for TC traveling westward or the TC turning point toward the shore over coastal waters being east- (west-)ward of average, making the yearly TC frequency increase (decrease). Besides, in the year of stronger (weaker) APO the offshore zonal wind vertical shear in the higher and lower troposphere becomes decreased (increased), convection is enhanced (enfeebled) and the SCS cyclonic (anticyclonic) circulation undergoes anomaly, all these favor (do not favor) the maintenance and intensification of the offshore TC activities.6) In late autumn and early winter (November– December), the possible mechanism for APO vigor impinging on the southern coastal TC activities acts to alter cold air strength, vertical shear of zonal winds, the low-level convergence intensity and steering airflow's direction. For strong (weaker) APO, the subtropical winter monsoon enhances (enfeebles), the subtropical high moves south- (north) and east-(west-) ward, the middle and upper troposphere of tropical western Pacific is under the control of anomalous easterly (westerly) winds, decreasing (increasing) the vertical shear of zonal westerlies over coastal waters of south China, while anomalies happen to SCS cyclonic (anticyclonic) circulation because of the lower anomalous northeast (southwest) winds over south coasts of China, plus declined(enriched) cold air in the SCS and western TWP, all these are favorable for intensifying (weakening) the TC maintenance and development in southern China coastal waters. Meanwhile, in years of intense (weak) APO, the steering airflow in the TWP middle troposphere is under the control of anomalous easterly (westerly) winds, a condition favorable (unfavorable) for TC traveling westward or the TC turning point towards the shore being west-(east-)ward of mean, causing a larger (smaller) number of TC in the coastal waters of southeast China.7) Despite the fact that the APO vigor in autumn and winter bears an intimate relation to El Ni?o or La Ni?a events , modelings show that these vigorous episodes excite no APO both in September– October and November– December.